1. Field of the Invention
The field of the invention is electronic controllers for controlling a machine, an assembly line or a commercial process. More particularly, the invention relates to programming a system controller with instructions based on fuzzy logic in addition to instructions based on Boolean logic.
2. Description of the Background Art
Programmable controllers are employed in many industrial and commercial applications to control the operation of various machines and processes. They fall into the broad category of real-time sampled data systems. In a certain maximum time, inputs are read, controlling equations are applied, and outputs are actuated. These controllers have employed Boolean or binary logic in which individual bits are evaluated as being either "true" or "false". Arithmetic functions, such as addition, subtraction, and multiplication have also been provided, along with instructions to transfer data bytes between a controller processor and I/O modules or to transfer data bytes within the memory of the controller.
Programmable controllers have been adapted to certain process control applications using analog I/O interface modules to receive analog signals from pressure and temperature sensors and to transmit signals to control analog outputs. For example an analog input module might receive an input signal in a range from 0.0 volts to +10 volts or from +10.0 volts to -10 volts, this range being scaled to the maximum and minimum values for temperature or pressure.
In recent years, a mathematical theory known as "fuzzy logic" has been the subject of a series of articles and papers by Lofti A. Zadeh. See, for example, Zadeh, L., "Fuzzy Logic" IEEE Computer, April, 1988, pages 83-93.
Fuzzy logic is not imprecise as the expression might suggest. It is, in fact, a mathematical theory based on a combination of multivalued logic, probability theory and expert system theory and does incorporate an element of mathematical precision. However, fuzzy logic allows answers or outputs which we might called "shades of gray", neither true nor false, but somewhat true or somewhat false at the same time. These shades of gray are also known as degrees of truth or grades of membership.
An example of an expert system of the general type utilized in a fuzzy logic system is disclosed in Hardy et al., U.S. Pat. No. 4,648,044 issued Mar. 3, 1987. Other references in the field are provided in Sakurai et al., U.S. Pat. No. 5,012,430 and in Self, K., "Designing with Fuzzy Logic", IEEE Spectrum, November, 1990, pages 42-44 and 105.